1. Field of the Invention
The present invention relates to a display device, and more particularly to a liquid crystal display device formed by a thin film semiconductor element.
2. Description of the Related Art
In recent years, portable phones are becoming more widely used in accordance with the advancement of communication technology, and it is expected that moving images or more amount of information will be transferred between them. Meanwhile, personal computers for mobile use that are light in weight are produced, and information terminals such as a PDA originated in an electronic data book are manufactured and also becoming widespread in the society. As a result of the development of display devices, most of those portable information devices are provided with flat panel displays such as a liquid crystal display device.
Among the active matrix liquid crystal display devices, a liquid crystal display device having a thin film element such as a low-temperature polysilicon thin film transistor (the thin film transistor is hereinafter referred to as a TFT) is advanced toward commercialization in recent years. By using the low-temperature polysilicon TFTs, not only pixels but a signal driver circuit can also be integrated in the periphery of the pixels. Therefore, it is quite advantageous in downsizing and achieving high resolution of the display, thus the further spread thereof is expected.
On the other hand, portable information devices are required to be equipped with not only a visible display function but other output functions such as an audio output function in particular. Displaying an image with audio makes the image more effective and amusing.
There are two types of audio signals for an audio output, which are an analog audio signal and a digital audio signal. The analog audio signal directly obtains audio by a microphone or an audio device of analog output, while the digital audio signal obtains signals from digital audio devices such as a CD, an MD, and a DVD.
Moreover, an electronic device such as an IC recorder using a semiconductor memory such as a memory card other than the aforementioned CD, MD and the like are becoming to be widely used as a digital audio signal source and expected to be spread further.
FIG. 6 is a block diagram showing a process of converting a digital audio signal into an analog audio signal. Most of the CD players and the like have optical outputs, which inputs an optical signal to an optical sensor 601 through fiber optics. The optical sensor 601 converts an optical signal into an electronic signal. The electronic signal is defined by CP-1201 standard of EIAJ and the like. Further, this signal is decoded into serial digital audio data by a demodulator 602 as shown in FIG. 9. There are three types of the decoded electronic signals as for the CD, which are a base clock of 1.411 MHz, an LR clock of 44.1 KHz, and digital audio signal data. The digital audio signal data is transferred to a serial-parallel converter circuit 603 in sequence from an MSB. The serial digital audio data is converted into parallel digital audio data by the serial-parallel converter circuit 603 and inputted to a D/A converter circuit 604 and converted into an analog audio signal.
FIG. 5 is an example of a conventional audio signal processing circuit. A D/A converter circuit to speaker driving terminals are shown in FIG. 5. The audio signal processing circuit comprises a D/A converter circuit 501, a D/A converter buffer circuit 502, a preamplifier 503, a speaker driving non-inverted amplifier 504, a speaker driving inverted amplifier 505, speaker driving terminals 506 and 507, coupling capacitors 508, 509, and 510, filter capacitors 511 and 512, input resistors 514 and 525, a preamplifier feedback resistor 516, a resistor 517, speaker drive amplifier gain setting resistors 518 to 520, a D/A converter circuit power supply 521, a bias power supply 522, an analog-digital switch 523, and an analog signal input terminal 524.
Hereinafter described is an operation of the audio signal processing circuit. The parallel digital audio signal is converted into an analog audio signal by the D/A converter circuit 501 and inputted to the switch 523 through the D/A converter buffer circuit 502 and the coupling capacitor 508. On the other hand, an analog audio signal is inputted from the input terminal 524 to the switch 523 through the coupling capacitor 509. The switch 523 selects either the output signal from the D/A converter buffer circuit 502 or the analog input signal, and the selected signal is inputted to the preamplifier 503. The preamplifier 503 amplifies the signal using the gain determined in response to the proportion of the preamplifier feedback resistor 516 and the resistor 515. Subsequently, an output signal of the preamplifier 503 is amplified by the speaker driving non-inverted amplifier 504 and outputted to the speaker driving terminal 506. Here, an example of a driving method of BTL (Bridged Trans Less) in which opposite-phase signals are applied to the both terminals of the speaker is described. An output of the speaker driving terminal 506 is inverted by the speaker driving inverted amplifier 505 and outputted to the speaker driving terminal 507.
FIGS. 3A and 3B are a top plan view and a side view of a conventional display device. A pixel portion 308, a source signal driver circuit 305, and a gate signal driver circuit 306 are formed by TFTs on an insulating substrate 301. Further, a sealing member 304 is sandwiched between the insulating substrate 301 and the opposing substrate 302 for sealing liquid crystal and electronic signals are supplied to the liquid crystal display device externally via an FPC (Flexible Printed Circuit) 303. Moreover, a TEG 312 for verifying characteristics and alignment marks 310 and 311 for aligning the opposing substrate 302, a panel ID 309 and the like are provided beside the connection portion with the FPC on the panel.
Further, another example of a conventional display device which is different from the aforementioned one is shown in FIGS. 2A and 2B. In FIG. 3A, TFTs surrounded with the sealing member 304 are only of the pixel portion 308, while a source signal driver circuit, a gate signal driver circuit, and a signal processing circuit are surrounded with a sealing member in FIG. 2A. A pixel portion 208, a source signal driver circuit 205, a gate signal driver circuit 206, and a signal processing circuit 207 are formed by TFTs on an insulating substrate 201. Further, a sealing member 204 is sandwiched between the insulating substrate 201 and an opposing substrate 202 for sealing liquid crystal and electronic signals are supplied to the liquid crystal display device externally via an FPC 203. Moreover, a TEG 212 for verifying characteristics and alignment marks 210 and 211 for aligning the opposing substrate 202, a panel ID 209 and the like are provided beside the connection portion with the FPC on the panel. A conventional technology can be referred in the following document (For example, Patent Document 1)
[Patent Document 1]
Japanese Patent Laid-Open No. 2000-292805